Through the years a number of recording devices have been created which provide a graphic output under computer control. Such devices include both graphic recorders and photoplotters in which a moveable recording element carriage is moved back and forth along a first path while a recording media is moved back and forth along a second path. In most instances, the motion paths for the media and the recording element carriage are perpendicular to each other.
In graphic recorders, one or more recording elements such as pens, are carried by the carriage and supported above the media. Means are provided for lowering one of the pens into contact with the media during times in which recording upon the media is desired.
The recording operation takes place in response to computer control which operates the recording element carriage and media movement in a coordinated fashion to produce the desired recorded image upon media. In most system, the recording element carriage is supported by one or more roller and track mechanisms which permit the free movement of the carriage along the carriage path. Usually, the recording element carriage is coupled to a continuous band drive system in which a pair of opposed rollers are supported at each end of the carriage path and a flexible band encircles the rollers and is supported in tension. At least one of the rollers is coupled to a drive motor which provides movement of the carriage band which in turn moves the carriage along the carriage path.
The media is positioned upon a carriage bed beneath the pen carriage path and is secured and driven by a plurality of rollers which are coupled to computer controlled drive means within the recorder. While several different configurations for the media drive means have been utilized, the most common system provides a pair of media roller shafts positioned above and below the media and spaced from the pen contact line for the pen carriage. The shafts each carry two or more rollers generally formed of a resilient material such as rubber or plastic which captivate the media and roll it back and forth across the carriage bed during the recording operation. A platen or recording surface is positioned beneath the pen carriage path and supports the media against the recording pen. In many instances, the platen defines an elongated groove or recess extending across the platen beneath the point of pen contact with the media. The recess provides some cushion for the recording pens and accommodates variations in recording pen extension. Thus, the media drive rollers are offset from the portion of the media in contact with the recording pens. This offset configuration is utilized to permit the recording pen carriage to move freely above the media along the recording line or path. However, the use of this offset configuration often produces problems in the recording process itself. For example, rapid media movement in the direction from the roller location toward the recording pen contact line often causes buckling of the media. In addition, the offset between the media drive rollers and the recording line necessitates a greater border upon the media at each end in the direction of media motion. Further, the offset imposes certain limitations upon the system as to recording pen pressure and pen writing characteristics as well as recording speeds.
While such offset media drive systems have met with some success in the marketplace, the limitations upon recorder performance which they impose have, in the wake of recent advances in the development of the remainder of the graphic recording system, become a source of significant limitation of overall graphic recorder performance. There arises, therefore, a need in the art for an improved, more effective system of media motion for graphic recorders which avoids imposing the above-described limitations.